Stress inversion methods: are they based on faulty assumptions?

Stress inversion methods employed by structural geologists for estimating a regional stress tensor from populations of faults containing slickenlines rely on the basic assumption that slip on each fault plane occurs in the direction of maximum resolved regional shear stress. This premise ignores directional differences in fault compliance caused by fault shape, the Earth's surface or frictional anisotropy of the fault itself. It is also assumed that the regional stress field is homogeneous in space and time. Thus, perturbations in the local stress field caused by such things as material heterogeneities near the fault and mechanical interaction with nearby faults are not considered. Regional stresses may exercise dominant control on the slip direction; however local factors may perturb this field. We show how differences in fault compliance and local stress perturbations can result in a measureable difference between the direction of resolved shear stress and the direction of fault slip. Numerical modeling of common fault geometries in an elastic half space provides a means for evaluating the magnitude of this difference. We illustrate a few examples of geological circumstances under which the inversion techniques should be reliable, and a few where errors related to violations of the basic assumptions exceed those inherent to the data gathering and inverse techniques.